Waste disposal process for spent wood-pulping liquors

ABSTRACT

WASH WATER AND DIGESTER LIQUOR FROM A WOOD PULPING OPERATION ARE HEATED AT A TEMPERATURE IN THE RANGE OF 450*F. TO 700*F. UNDER AUTOGENEOUS PRESSURES OF ABOUT 1000 TO ABOUT 3000 P.S.I.G. FOR A PERIOD OF 0.5 MINUTE TO 6 HOURS TO FORM COKE AND A PURIFIED EFFLUENT. THE COKE IS SEPARATED FROM THE EFFLUENT AND DEWATERED. LIQUID FROM THIS DEWATERING STEP IS COMBINED WITH THE EFFLUENT AND DEPRESSURED TO FLASH VAPORIZE AND CONCENTRATE THE COMBIMED STREAM TO A SOLUBLE CHEMICALS CONCENTRATION SUITABLE FOR RECONSTITUTION AND RECYCLE TO THE PULPING OPERATION. COKE PRODUCED IN THE PROCESS MAY BE BURNED AS PLANT   FUEL AND THE ASH RECOVERED AND WORKED FOR CHEMICAL VALUES. THE LIQUID STREAM IS RECONSTITUTED FOR PULPING PURPOSES BY THE ADDITION THERETO OF PULPING CHEMICALS DERIVED, IN PART, BY BURNING THE COKE PRODUCED IN THE PROCESS.

Dec. 5, 1.72 w. F. FRANZ ErAL WASTE DISPOSAL PROCESS FOR SPENT WOOD-PULPING LIQUQRS Filed Oct. 9, 1970 SEPA- RATOR 40 L|QU|D FROM COKE DE WATERING spam cooxms uoum 36 DIGESTER LIQUOR EFFLUENT A 8 /37 P COKER Z 6353 PULP WASH 24 WASHING WATER 5 l2 Y um 22 .L

BLOW DOWN. 0'

UNIT

United States Patent 3,705,077 WASTE DISPOSAL PROCESS FOR SPENT WOOD-PULPING LIQUORS William F. Franz, Gardiner, Howard V. Hess, Glenham,

and Edward L. Cole, Fishkill, N.Y., assignors to Texaco Inc, New York, NY.

Filed Oct. 9, 1970, Ser. No. 79,385 Int. Cl. D21c 11/00 US. Cl. 162-30 6 Claims ABSTRACT OF THE DISCLOSURE Wash water and digester liquor from a wood pulping operation are heated at a temperature in the range of 450 F. to 700 F. under autogeneous pressures of about 1000 to about 3000 p.s.i.g. for a period of 0.5 minute to 6 hours to form coke and a purified efiluent. The coke is separated from the efiluent and dewatered. Liquid from this dewatering step is combined with the effluent and depressured to flash vaporize and concentrate the combined stream to a soluble chemicals concentration suitable for reconstitution and recycle to the pulping operation. Coke produced in the process may be burned as plant fuel and the ash recovered and worked for chemical values. The liquid stream is reconstituted for pulping purposes by the addition thereto of pulping chemicals derived, in part, by burning the coke produced in the process.

'FIELD OF THE INVENTION This invention relates to a waste disposal process for spent wood-pulping liquors which results in the production of a liquid effluent containing recovered chemicals suitable for reconstitution and recycle to the pulping process.

DESCRIPTION OF THE PRIOR ART The kraft or sulfide process and the sulfite process are currently responsible for the bulk of the paper pulp production. In the knit process wood chips are digested at an elevated temperature with an aqueous solution of sodium hydroxide, sodium sulfide, and inactive sodium carbonate. Pulp and cooking liquor are removed from the digester, the pulp is separated from the cooking liquor, and the pulp is water washed to remove the occluded waste liquor. The cooking liquor or black liquor is combined with all or a portion of the wash liquor. The recovered liquors are then evaporated in multiple-effect evaporators to a solids content of about 50 percent. The concentrated liquor may be further concentrated by direct contact with hot flue gases in a direct contact evaporator prior to being burned in a recovery furnace. In the recovery furnace all the organic matter is destroyed and the inorganic sulfur including make-up salt cake is reduced to sodium sulfide. The smelt from the recovery furnace, containing sodium carbonate and sodium sulfide is dissolved in water and reconstituted with lime. The resulting white liquor is returned to the digester for the cooking step. By these procedures chemicals are recovered but at the great cost of evaporating all or substantially all of the water used in cooking and washing.

Generally, it has not been economically feasible to recover chemicals from sulfite processing of pulp except for sulfur dioxide which is recovered as relief gas. However, recent pollution abatement procedures have encouraged the recovery of chemicals and have resulted in the use of evaporators in procedures similar to that employed in the kraft process. These procedures are expensive since large amounts of water must be evaporated.

Patented Dec. 5, 1972 SUMMARY OF THE INVENTION In accordance with the present invention, it has been found that the wash water and digester liquor from the wood-pulping operation after coking at a temperature in the range of 450 to 700 F. under autogenous pressures of about 1000 to about 3000 p.s.i.g. for a period of 0.5 minute to 6 hours to form coke and a purified effiuent, can have the coke and purified etfiuent thus produced separated; the coke dewatered and the liquid from this dewatering step can be combined with the efiluent from the coking step and depressured to a range of between 500 to 0 p.s.i.g. to vaporize water and concentrate the combined stream to a solids content suitable for chemical reconstitution and recycle to the pulping process. Some of these pulping chemicals can be obtained by burning the coke produced in the coking step.

An important feature of the present invention is that the energy contained in the high temperature high pressure coker effluent serves to vaporize, and thereby remove, excess water added to the system with other pulping waste streams.

The process will be better understood by reference to the accompanying figure illustrating diagramatically a preferred embodiment of the present invention as applied to the treatment of a waste wood-pulping liquor in a system having a flash vaporization stage to concentrate the exit water streams prior to their being reconstituted with chemicals for use in the pulping operation.

'Essentialy, liquid phase coking replaces the evaporator section of a pulping liquor recovery-disposal system. The chemicals to be recovered are divided between the two efliuent streams resulting from the coking operation, the main stream and the liquid obtained by dewatering the coke. The coke may be burned for steam generation and chemicals obtained from this ash in known manner.

Referring to the drawing, the system of the invent on is seen to be integrated with that part of a paper maklng plant where subsequent to separation of the pulp from the spent cooking liquor, the pulp is washed to remove residual cooking liquor. Wash water enters the pulp washing unit 10 through line 12 and emerges through line 14. The pulp reaches the pulping washing unit 10 from pulping digester 16 through line 8. Spent cooking liquor from the digesters passes through line 18 and joins with the wash liquors at the junction of lines 14 and 18. The combined liquids are pumped by pump 20 under a pressure of about 1050 p.s.i.g. to coking unit heater 22 where they are coked at, for example, a temperature of 550 F. and a pressure of 1050 p.s.i.g. The efliuent thus produced then passes through line 24. into coker hold drum 26 Where coking is completed and the coke settles to the bottom of the vessel. The coker hold drum retention time depends on the type of spent pulping liquor being processed, the operating conditions employed in the pulping digesters, the type of wood being processed in the pulping operation as well as the operating temperature employed in the coking process. Spent kraft pulping liquors have been found to require about two hours residence time at coking conditions while sulfite pulping liquors may be processed using residence times of three to twenty minutes, depending on the other variables listed above. Thus, a preferred range of residence times might be from three minutes to two hours.

The coke slurry passes through the bottom of the drum 26 and through line 28 into blow-down unit 30 where it is dewatered. The separated coke passes the bottom of unit 30 through line 32 for transfer to a furnace. Liquid eflluent passes from the top of drum 26 through line 37 and is combined at point 36 with liquid from the coke dewatering step. The combined liquid stream is then depressurized through the throttling valve 38 to vaporize the required amount of water, which is dictated by the amount thereof present and the desired concentration of the cooking liquor which is to be made therefrom.

The pressure reduction required in any case is deterranging between 0.5 minute to 6 hours to form coke and a purified efiluent containing pulping chemicals dissolved therein;

separating said coke from said eflluent;

mined by a heat balance around the vaporization section. dewatering said coke; Sufiicient pressure reduction must be taken to provide the combining said efiluent with water obtained by said enthalpy of vaporization of the excess water contained in dewatering; the coker feed. Excess water is present because of the depressuring the resulting stream sufi'iciently to provide inclusion of pulp wash water and/or other Waste streams enthalpy necessary to vaporize and concentrate said with the spent pulping liquor in the feed to the liquid stream so as to render same suitable for reconstituphase coking unit. tion and recycle as pulping liquor in said process.

Separation of the vapor from the liquid takes place 2. The process as defined in claim 1, comprising burnin separator 40. Liquid containing recovered chemicals ing said coke, thereby producing ash and steam and refor reconstitution and recycle to digesters flows out covering pulping chemicals from said ash. through line 42 and steam which can be used in the The Process as defined in Claim 1, wherein Said process passes out of the unit through line 44. depressuring is effected down to the range of between 500 The experimental data tabulated below show results, p.s.i.g. and 0 p.s.i.g. including dissolved solids in the coker liquid eflluent, ob- 4. The process as defined in claim 1, wherein said tamed with the various types of spent wood pulping p p Waste Stffiam is P p Wash Waterliquors processed. 5. The process as defined in claim 2, comprising adding TABLE I [Spent krait pulping liquor colred at 600 F., 1,375 p.s.i.g. with 2 hours hold time] Spent liquor charge Coker liquor efiluent y coke Dis- D15. solved solved solids, Ash, Yield, solids, Ash, e d, weight COD, weight weight weight COD, weight weight weight percent g./l. percent pH percent percent g.[l. percent pH percent percent 24. s 175 5. 97 12 90. 0 10. a 01.9 5. 3s 9. s 3. 9 8.35

TABLE II [Spent ammonia base sulfite pulping liquor coked at 550 F., 1,000 p.s.l.g. with 20 minutes hold time] Spent liquor charge Coker liquor efiiuent Dry coke Dis- Dissolved solved sollds Ash, Yield, solids, Ash, Yield, Ash, weight COD, weight weight weight COD, weight weight wlight percent g./l. percent pH percent percent g./l. percent pH percent percent TABLE III [Spent sodium base sulfite pulping liquor coked at 550 F., 1,000 p.s.i.g. with 2 hours hold time] Spent liquor charge Ooker liquor efliuent Dry coke Dis- Dissolved solved solids, Ash Yield, solids, Ash, Yield, Ash, weight COD, weight Weight weight COD, weight weight weight percent g./l. percent pH percent percent gJl. percent pH percent percent An important advantage of the present process is that the major portion of the recovered chemicals is obtained in solution in the liquid coker efliuent. This solution is then used to dissolve the remainder of the chemicals recovered by burning the coke.

While the invention has been illustrated with the aforedescribed physical embodiments, these are exemplary only and the scope of the invention is limited onl by the subjoined claims.

We claim:

1. In a spent wood pulping liquor treatment process, the steps of:

mixing spent digester liquor with at least one pulping waste stream, coking the mixture at a temperature in the range of 450 to 700 F. under a pressure of about 1000 to about 3000 p.s.i.g. for a period of time said recovered pulping chemicals to said stream to reconstitute pulping liquor.

6. The process as defined in claim 1, wherein the coking time ranges from 3 minutes to two hours.

US. Cl. X.R. 162--36; 201-25 

